Construction of higher-ordered structures in alkylpyridinium-based polymers via π-conjugated anion pairing and humidity annealing

Abstract

In this study, π-conjugated anions were used as counterions in poly(4-vinyl-N-alkylpyridinium) (P4VC_xP⁺) to construct highly ordered ionic microstructures. Nuclear magnetic resonance, Fourier transform infrared, and ultraviolet–visible spectroscopies, and differential scanning calorimetry were employed to characterize the resulting ion-pair polymers. Furthermore, thin films were fabricated and subjected to humidity annealing above the glass transition temperature. Grazing-incidence small-angle X-ray scattering showed that humidity annealing promoted the formation of hexagonally packed cylindrical microdomains in P4VC_xP⁺ with π-conjugated anions. The π-conjugated anions increased the effective hydrophilic volume and facilitated the structural alignment via π–π and dipole–dipole interactions. Optical absorption analysis indicated that both pyridinium and π-conjugated anions exhibited end-on orientations and revealed the dynamic behavior of 7,7,8,8-tetracyanoquinodimethane radical anion, including dimerization and charge-transfer complex formation. Thus, π-conjugated ion pairs show significant promise for the controlled self-assembly and functional optical properties of polymeric materials.

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Introduction

Microstructures composed of multiple chemical species are widely employed in materials for ionic conduction, gas separation, and photonic applications, depending on the combination of their constituent elements.^1,2 Intermolecular interactions, including π–π interactions, electrostatic interactions, and van der Waals interactions, are effective for controlling the assembled structure of such multi-component systems. In polymer chemistry, microstructures are constructed by the segregation between polymer chains in block copolymers. By controlling the volume ratio of each block, self-assembled structures such as spheres, cylinders, gyroids and lamellae can be formed.³ Notably, our group has recently reported a higher-order structure through humidity annealing of comb-shaped homo(co)polymers composed of polar main chains with non-polar alkyl chains.⁴ Water is adsorbed onto the amide and ester main chains, thereby increasing the segregation force between the polar main chains and non-polar side chains, which leads to the formation of a lamellar structure. Subsequently, Terashima et al. reported that water annealing can be used to form higher-order structures in cationic polymers with nonpolar alkyl chains.^5,6 The counter anion plays a key role in enhancing the hydrophilicity of these polymers.

However, the influence of the interactions between counter anions or anions and cations on the higher-order structure of polymers has not been explored. In addition, the majority of higher-order structures reported to be induced by humid annealing are lamellar structures,^4–9 with only a few studies reporting the formation of more highly ordered cylindrical structures.^10,11 Ion-pair materials offer a notable advantage because different combinations can impart diverse functions and properties. We previously reported the synthesis of poly(4-vinyl-N-dodecylpyridinium) (P4VC₁₂P⁺) and introduced a fan-shaped anion as a counter anion to form a hexagonal columnar structure.¹² Furthermore, by incorporating a photo-responsive counter anion enabled photoisomerization and mass transfer upon ultraviolet (UV) irradiation.

Based on this background, the present study investigated two types of π-conjugated anions, 1,3-(bisdicyanomethylidene)indan anion (In⁻)¹³ and 7,7,8,8-tetracyanoquinodimethane radical anion (TCNQ˙⁻),^14–19 which are expected to promote the formation of higher-order structures and impart functional properties. The stable π-conjugated anion, In⁻, is produced through the deprotonation of indane. As evidenced in prior research, its significant dipole moment allows it to overcome electrostatic repulsion between identical charges, thereby facilitating the formation of dimers.^20,21 Similarly, TCNQ˙⁻ is known to facilitate structural control and modulate electrical conductivity when paired with suitable cations, as reported by Akutagawa et al.²² In this study, we aimed to construct highly ordered microstructures and assess the optoelectronic properties of In⁻ and TCNQ˙⁻ by introducing them as counter anions in cationic polymers. We synthesized poly(4-vinyl-N-decylpyridinium) (P4VC₁₀P⁺) and poly(4-vinyl-N-tetradecylpyridinium) (P4VC₁₄P⁺), incorporating In⁻ and TCNQ˙⁻ as counter anions and evaluated their higher-order structures and optical properties. Consequently, we successfully constructed a hexagonal columnar structure of ionic polymers comprising π-conjugated anions via humidity annealing for the first time.

Experimental

Materials and methods

The reagents were purchased from FUJIFILM Wako Pure Chemical, Kanto Chemical, TCI, MITSUBISHI Chemical, and Sigma-Aldrich, and used without further purification unless otherwise stated. The solvents, chloroform (CHCl₃, ≥99.0%), n-hexane, methanol (MeOH, ≥99.8%), ethanol (≥99.5%), tetrahydrofuran (THF, ≥99.0%) and N,N-dimethylformamide (DMF, ≥99.9%), were used without further purification. ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopies were investigated on JEOL ECX-500 500 MHz spectrometers using CDCl₃ and DMSO-d₆ as the internal standards. The phase transitions were measured on a differential scanning calorimetry using Hitachi SII EXSTAR DSC6220. UV-visible (vis)-near infrared (NIR) diffuse reflectance spectra were recorded on a JASCO V-750 spectrometer using a quartz plate.

Preparation of sample films. Film samples were prepared by spin–coating using 2 wt% solutions of P4VC_xP⁺–Br⁻, P4VC_xP⁺–In⁻ and P4VC_xP⁺–TCNQ˙⁻ (x = 10, 14) dissolved in CHCl₃, THF, and DMF/THF (5 : 1), respectively, onto the glass substrates under ambient conditions (600 rpm for 10 s, followed by 1000 rpm for 60 s). To remove the solvent, the films were annealed in vacuum at 50 °C for 1 h. Humidity annealing was performed in a sealed glass container with pure water at 60 or 65 °C in an oven for 0.5, 1, 3, 6, 12, and 24 h.

Differential scanning calorimetry (DSC). The phase transitions were measured on a differential scanning calorimetry (Hitachi SII EXSTAR DSC6220).

Grazing-incidence small-angle X-ray scattering (GI-SAXS). GI-SAXS measurements were performed using a NANO-Viewer (RIGAKU). Beams of 3.0 mm collimated Cu-Kα radiation (λ = 1.5418 Å) were used as an X-ray, and the camera was placed at a distance from the sample of 0.3646 m for GI-SAXS and 0.1146 m for GI-WAXS, respectively.

Thermogravimetry/differential thermal analysis (TG–DTA). TG–DTA measurements were carried out on a Hitachi SII EXSTAR TG/DTA 6200 under a dry nitrogen atmosphere. The temperature was ramped from 25 to 250 °C at 10 °C min⁻¹.

Gel permeation chromatography (GPC). Gel permeation chromatography (GPC) was performed using a PU-2080 Plus pump, a CO-2060 Plus column oven, and an RI-2031 Plus refractive index detector (all from Jasco, Japan), equipped with a GASTORR FG-42 degassing unit (FLOM, Japan). The column set consisted of Shodex KD-G, KD-806M, KD-806M, and SB-802.5 HQ columns. The measurements were carried out at 40 °C using DMF containing 10 mM LiBr as the eluent at a flow rate of 1.0 mL min⁻¹.

Synthesis of ionic polymer

Typical procedure of the synthesis of poly(4-vinyl-N-alkylpyridinium bromide) (P4VC_xP⁺–Br⁻). Poly(4-vinylpyridine) (P4VP) (Kanto Chemical, M_n = 5.11 × 10⁴, M_w = 1.04 × 10⁵M_w/M_n = 2.04) (0.30 g) and alkylbromide (14.4 mmol) were added to CHCl₃ (9 mL), and the mixture was heated at 60 °C for 3 days. The reaction mixture was poured into the cold hexane to obtain P4VC_xP⁺–Br⁻.
Poly(4-vinyl-N-decylpyridinium bromide) (P4VC₁₀P⁺–Br⁻). Pale yellow powder, 82% yield. ¹H NMR (500 MHz, CDCl₃, δ): 8.91 (2H, Ar–H), 7.99 (2H, Ar–H), 4.66 (2H, NCH₂), 2.04 (2H, NCH₂CH₂), 1.59–1.00 (14H, C₂H₄(CH₂)₇), 0.88 (3H, C₉H₁₈CH₃). IR (ATR, cm⁻¹): 3745, 3648, 3392, 3116, 3043, 2922, 2360, 2337, 2046, 1995, 1944, 1869, 1844, 1792, 1772, 1749, 1716, 1637, 1558, 1541, 1516, 1466, 1417, 1375, 1230, 1173.
Poly(4-vinyl-N-tetradecylpyridinium bromide) (P4VC₁₄P⁺–Br⁻). White powder, 80% yield. ¹H NMR (500 MHz, CDCl₃, δ): 8.98 (2H, Ar–H), 8.29 (2H, Ar–H), 4.79 (2H, NCH₂), 2.06 (2H, NCH₂CH₂), 1.46–1.09 (22H, C₂H₄(CH₂)₁₁), 0.88 (3H, C₁₃H₂₆CH₃). IR (ATR, cm⁻¹): 3735, 3648, 3566, 3523, 2920, 2850, 2363, 2163, 2331, 1992, 1942, 1869, 1844, 1792, 1749, 1716, 1697, 1647, 1558, 1541, 1518, 1508, 1420, 1375, 1338, 1174.

Typical procedure of the synthesis of poly[4-vinyl-N-alkylpyridinium 1,3-(bisdicyanomethylidene)indan anion] (P4VC_xP⁺–In⁻). An ethanol solution (100 mL) of P4VC_xP⁺–Br⁻ (1 mmol) was heated at 65 °C to complete dissolution, and an ethanol solution (100 mL) of Na⁺–In⁻ (264.2 mg, 1 mmol) was added. The mixture was refluxed overnight. The mixture was then allowed to stand at room temperature. The precipitate was collected by filtration and washed with ethanol until the filtrate became colorless, thus affording P4VC_xP⁺–In⁻.
Poly[4-vinyl-N-decylpyridinium 1,3-(bisdicyanomethylidene)indan anion] (P4VC₁₀P⁺–In⁻). Deep blue powder, 78% yield. ¹H NMR (500 MHz, DMSO-d₆, δ): 8.91 (2H, Ar–H), 7.99 (2H, Ar–H), 7.91–7.88 (m, 2H), 7.41–7.39 (m, 2H), 5.65 (1H, s), 4.45 (2H, NCH₂), 2.09 (2H, NCH₂CH₂), 1.48–1.04 (14H, C₂H₄(CH₂)₇), 0.84 (3H, C₉H₁₈CH₃). IR (ATR, cm⁻¹): 3735, 3689, 3649, 3568, 3545, 3523, 2981, 2922, 2852, 2362, 2330, 2185, 1992, 1967, 1869, 1844, 1792, 1772, 1749, 1716, 1647, 1541, 1516, 1508, 1473, 1456, 1417, 1396, 1362, 1338, 1178, 690, 669, 617.
Poly[4-vinyl-N-tetradecylpyridinium 1,3-(bisdicyanomethylidene)indan anion] (P4VC₁₄P⁺–In⁻). Deep blue powder, 66% yield. ¹H NMR (500 MHz, DMSO-d₆, δ):8.90 (2H, Ar–H), 8.30 (2H, Ar–H), 7.90–7.88 (m, 2H), 7.41–7.38 (m, 2H), 5.66 (1H, s), 4.50 (2H, NCH₂), 2.08 (2H, NCH₂CH₂), 1.48–1.05 (22H, C₂H₄(CH₂)₁₁), 0.84 (3H, C₁₃H₂₆CH₃). IR (ATR, cm⁻¹): 3735, 3689, 3649, 3566, 3545, 3523, 2981, 2920, 2850, 2360, 2332, 2183, 1967, 1869, 1844, 1792, 1772, 1749, 1716, 1697, 1647, 1541, 1522, 1508, 1473, 1458, 1419, 1396, 1362, 1338, 1177, 688, 669, 617.

Typical procedure of the synthesis of poly(4-vinyl-N-alkylpyridinium 7,7,8,8-tetracyanoquinodimethane radical anion) (P4VC_xP⁺–TCNQ˙⁻). An ethanol solution (200 mL) of P4VC_xP⁺–Br⁻ (1 mmol) was heated at 65 °C to completely dissolve, and ethanol solution (200 mL) of Li⁺–TCNQ˙⁻ (211.1 mg, 1 mmol) was added. The mixture was stirred for 15 min at 65 °C under light-shielding conditions. After confirming for the complete mixing, the mixture was left quietly returned to room temperature. The precipitate was collected by filtration and washed with ethanol until the filtrate became colourless, affording P4VC_xP⁺–TCNQ˙⁻.
Poly(4-vinyl-N-decylpyridinium 7,7,8,8-tetracyanoquinodimethane radical anion) (P4VC₁₀P⁺–TCNQ˙⁻). Deep blue powder, yield: 81%, IR (ATR, cm⁻¹): 3520, 3391, 3123, 3045, 2952, 2233, 2175, 2122, 1988, 1942, 1919, 1868, 1844, 1772, 1749, 1697, 1684, 1647, 1558, 1540, 1506, 1458, 1417, 1361, 1338, 1319, 1270, 1170, 1102, 1017, 827, 659.
Poly(4-vinyl-N-tetradecylpyridinium 7,7,8,8-tetracyanoquinodimethane radical anion) (P4VC₁₄P⁺–TCNQ˙⁻). Deep blue solid, yield: 65%, IR (ATR, cm⁻¹): 3523, 3124, 3051, 2952, 2921, 2852, 2168, 2123, 1942, 1919, 1869, 1844, 1792, 1772, 1749, 1704, 1684, 1637, 1593, 1541, 1506, 1466, 1437, 1362, 1319, 1271, 1171, 1103, 1047, 1022, 827, 769, 665.

Results and discussion

Synthesis

Ion pairs of poly(4-vinyl-N-alkylpyridinium) cations (P4VC_xP⁺, x = 10 or 14) with either In⁻or TCNQ˙⁻ were prepared via an ion exchange reaction using the bromide salt of P4VC_xP⁺ (P4VC_xP⁺–Br⁻) and either the sodium salt of In⁻ (Na⁺–In⁻) or lithium salt of TCNQ˙⁻ (Li⁺–TCNQ˙⁻) (Fig. 1). ¹H and ¹³C NMR, along with FT-IR spectroscopy, were used to characterize the synthesized ion-pair polymers. UV-vis absorption spectra were recorded in MeOH and DMF to confirm the successful ion exchange. The absorption spectra of P4VC_xP⁺–Br⁻ (x = 10 or 14) in MeOH showed a band at approximately 230–240 nm and 260 nm, which were attributed to the absorption from the pyridine group (Fig. 2(a)). In contrast, the ion-exchanged polymers exhibited absorption bands corresponding to π-conjugated anions at 582 nm for In⁻ (Fig. 2(b)) and 410 and 850 nm for TCNQ˙⁻ (Fig. 2(c)). Furthermore, the degree of anion incorporation was determined by method-specific spectroscopy: for In⁻, incorporation was quantified from the ¹H NMR integral ratio between the CH proton at the 2-position of the indan anion and the methylene protons adjacent to the quaternary nitrogen of the polymer backbone (Fig. S4 and 5); for TCNQ˙⁻, incorporation was estimated from UV–vis absorption analysis because the radical anion is not observable by NMR (Table S1). Using these procedures, the incorporation ratios were 58% (P4VC₁₀P⁺–In⁻), 64% (P4VC₁₄P⁺–In⁻), 59% (P4VC₁₀P⁺–TCNQ˙⁻), and 49% (P4VC₁₄P⁺–TCNQ˙⁻).

Fig. 1 Preparation of ionic polymers comprising poly(4-vinyl-N-alkylpyridinium) cations (P4VC_xP⁺) and 1,3-(bisdicyanomethylidene)indan anion (In⁻) or 7,7,8,8-tetracyanoquinodimethane radical anion (TCNQ˙⁻).

Fig. 2 UV–vis absorption spectra of (a) P4VC_xP⁺–Br⁻ in MeOH, (b) P4VC_xP⁺–In⁻ in DMF, and (c) P4VC₁₀P⁺–TCNQ˙⁻ in DMF (10⁻⁵ M).

Thermal properties

DSC was employed to examine the thermal properties of ion pairs in their bulk states. The DSC thermograms show baseline shifts corresponding to glass transition temperatures (T_g) of 50, 45, 57, 56, 58, and 49 °C for P4VC₁₀P⁺–Br⁻, P4VC₁₄P⁺–Br⁻, P4VC₁₀P⁺–In⁻, P4VC₁₄P⁺–In⁻, P4VC₁₀P⁺–TCNQ˙⁻, and P4VC₁₄P⁺–TCNQ˙⁻, respectively (Fig. S7). The incorporation of π-conjugated anions restricted the mobility of the polymer chains, resulting in a slight increase in the T_g. The absence of melting points for the long alkyl chains suggests that they did not crystallize within the measured temperature range, which is consistent with previous studies.^4,23 On the other hands, the TG–DTA curves showed no significant weight loss below 100 °C, confirming the absence of residual solvent molecules (Fig. S8).

X-ray diffraction measurement

Thin films of P4VC_xP⁺–Br⁻, P4VC_xP⁺–In⁻, and P4VC_xP⁺–TCNQ˙⁻ (x = 10, 14) were prepared by spin-coating using chloroform, THF, and a DMF/THF (5 : 1) solution for P4VC_xP⁺–Br⁻, P4VC_xP⁺–In⁻, and P4VC_xP⁺–TCNQ˙⁻, respectively. The films were subjected to annealing for durations of 0, 0.5, 1, 3, 6, 12, and 24 h under conditions of saturated humidity at 60 °C (for P4VC_xP⁺–Br⁻ and P4VC_xP⁺–TCNQ˙⁻) or 65 °C (for P4VC_xP⁺–In⁻), which is above the T_g. Subsequently, the structural characteristics of the films were analyzed using GI-SAXS (Fig. 3 and Table 1). Before humidity annealing, P4VC₁₀P⁺–Br⁻ and P4VC₁₄P⁺–Br⁻ exhibited Debye–Scherrer rings in the 2D diffraction profiles at d = 3.0 and 4.0 nm, respectively (Fig. 3(a)(i) and (b)(i)). This scattering can be attributed to aggregation of the alkyl side chains.²³ In contrast, a humidity-annealed film of P4VC₁₀P⁺–Br⁻ exhibited spot-like diffraction peaks at q = 2.55 nm⁻¹ (d = 2.46 nm) in two distinct directions: the out-of plane direction and a direction inclined approximately 60° from the surface normal. These peaks indicated the formation of hexagonally packed cylindrical structures oriented parallel to the substrate plane.²⁴ The lattice constant was determined to be a = 2.8 nm, which is approximately twice the extended chain model of N-decylpyridinium (L = 1.4 nm). These findings suggested the formation of a columnar structure, in which the alkyl side chains on the pyridinium groups were oriented outward from the polymer backbone (Fig. 4). The phase morphology and domain spacing of P4VC₁₀P⁺–Br⁻ was further observed by scanning transmission electron microscopy (STEM, Fig. S22). In contrast, humidity annealing had no effect on the diffraction peak in P4VC₁₄P⁺–Br⁻ (Fig. 3(b)(ii)). Strong hydrophilic-hydrophilic interactions have been reported to occur in long side chains, which hinder the segregation force.⁴ Therefore, the tetradecyl side chains likely remained in the aggregated structure of the humidity-annealed film. These results indicated that the length of the alkyl groups plays a crucial role in the formation of the microphase-separated structure of P4VC_xP⁺. By utilizing the higher-order structures of these cationic polymers, π-conjugated anions such as In⁻ and TCNQ˙⁻ are anticipated to be spatially organized. In the GI-SAXS measurements of P4VC₁₀P⁺–In⁻ thin films, a Debye–Scherrer ring (q = 2.34 nm⁻¹) was observed before humidity annealing, whereas distinct diffraction peaks at q = 2.69 nm⁻¹ appeared after humidity annealing. These findings indicated a structural transition from alkyl nanodomains to a hexagonal columnar structure induced by humidity annealing, which is similar to that observed for P4VC₁₀P⁺–Br⁻. A similar transition was observed for P4VC₁₀P⁺–TCNQ˙⁻. These finding indicate that even when the counter anion was altered from Br⁻ to bulkier species such as In⁻ or TCNQ˙⁻, the formation of higher-order structures in P4VC₁₀P⁺ was not impeded.

Fig. 3 GI-SAXS patterns of (a) P4VC₁₀P⁺–Br^–, (b) P4VC₁₄P⁺–Br^–, (c) P4VC₁₀P⁺–In^–, (d) P4VC₁₄P⁺–In^–, (e) P4VC₁₀P⁺–TCNQ˙^–, and (f) P4VC₁₄P⁺–TCNQ˙^–: (i) before annealing and (ii) after humidity annealing for 24 h.

Fig. 4 Schematic representation of the higher-order structural changes in P4VC_xP⁺–anion⁻ caused by humidity annealing. The positions of counter anions are depicted schematically, as their exact locations cannot be determined experimentally.

Table 1 Summary of GI-SAXS measurements before and after humidity annealing

Samples	Annealing	q/nm^−1 (d/nm)	Morphology
q values correspond to the peak positions observed in the GI-SAXS patterns shown in Fig. 3.
P4VC10P^+–Br^−	0 h	2.09 (3.01)
		6.55 (0.95)
	24 h	2.55 (2.46)	Hexagonal (a = 2.8 nm)
		7.35 (0.85)
P4VC14P^+–Br^−	0 h	1.55 (4.04)
		6.49 (0.97)
	24 h	1.88 (3.34)
		7.41 (0.85)
P4VC10P^+–In^−	0 h	2.34 (2.68)
	24 h	2.69 (2.40)	Hexagonal (a = 2.7 nm)
P4VC14P^+–In^−	0 h	2.00 (3.15)
	24 h	2.11 (2.98)	Hexagonal (a = 3.4 nm)
P4VC10P^+–TCNQ˙^−	0 h	2.03 (3.09)
		6.55 (0.96)
	24 h	2.48 (2.53)	Hexagonal (a = 2.9 nm)
		16.13 (0.39)
P4VC14P^+–TCNQ˙^−	0 h	1.73 (3.63)
		6.72 (0.93)
	24 h	2.02 (3.12)	Hexagonal (a = 3.6 nm)
		15.60 (0.40)

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Notably, P4VC₁₄P⁺, which did not exhibit higher-order structure formation under humidity annealing with Br⁻, successfully formed a hexagonal columnar structure when In⁻ or TCNQ˙⁻ was used as the counter anion. The large π-conjugated anions increase the hydrophilic volume. Simple calculations of Br⁻ and π-conjugated anions indicated that the size of the π-conjugated anions was approximately four times larger than that of Br⁻ (Fig. S23). Consequently, P4VC₁₄P⁺ polymers self-assembled into hexagonal cylinders, similar to P4VC₁₀P⁺, due to the segregation between the tetradecyl side chains and the hydrophilic main chain, in combination with π-conjugated anions. In addition, π-conjugated anions are expected to exhibit attractive forces owing to dipole–dipole and π–π interactions. Indeed, the 1D GI-WAXS profile exhibited a diffraction peak at q = 14–16 nm⁻¹ for the TCNQ˙⁻ salt, which can be attributed to the π–π stacking of TCNQ˙⁻ (Table 1, Fig. S17 and S19). In summary, the GI-SAXS data revealed that all polymers initially exhibited random structures with the formation of alkyl nanodomains. Following humidity annealing, P4VC₁₀P⁺ with a decyl alkyl chain forms a horizontally oriented hexagonal columnar structure on the substrate, regardless of the counter anions. In contrast, P4VC₁₄P⁺ with a tetradecyl alkyl chain formed a hexagonal structure by humidity annealing only when it had a relatively large and highly hydrophilic π-conjugated anion. Interestingly, the lattice constants (a) of P4VC₁₄P⁺–In⁻ (a = 3.4 nm) and P4VC₁₄P⁺–TCNQ˙⁻ (a = 3.6 nm) were greater than those of P4VC₁₀P⁺–In⁻ (a = 2.7 nm) and P4VC₁₀P⁺–TCNQ˙⁻ (a = 2.9 nm), reflecting the influence of the alkyl chain length.

Optical properties in film state

UV–vis absorption spectroscopy was used to characterize the optical properties of the structured films and investigated the orientation of the polymer and π-conjugated anions (Fig. 5 and S27–30). In P4VC₁₀P⁺–Br⁻, an absorption band at approximately 230–280 nm from pyridine groups decreased with humidity annealing, which suggests that a part of pyridine rings adopted an end-on orientation relative to the substrate by formation of cylinders (Fig. 5a and d). This orientation is consistent with the proposed structure in which main chains are oriented parallel to the substrate plane. Although P4VC₁₄P⁺–Br⁻ did not form a hexagonal structure, similar changes in the absorption spectrum were observed, indicating an end-on orientation of the pyridine groups relative to the substrate. Notably, similar trends were observed for the absorption bands associated with the π-conjugated anions. For P4VC_xP⁺–In⁻, the absorption peak position did not change, however, the intensity decreased following the formation of hexagonally aligned cylindrical structures (Fig. 5b). In addition, these decreases in the absorption bands were recovered by collapsing the cylindrical structure into a randomly oriented film through vapor annealing using THF (Fig. S29). These results indicated that the π-conjugated anions also adopted an end-on conformation within the cylinders (Fig. 5d). In addition, the spectrum of P4VC_xP⁺–TCNQ˙⁻ exhibited complicated changes following humidity annealing (Fig. 5c). Prior to annealing, the films exhibited multiple absorption peaks across a wide range from the visible to the NIR region. The absorption bands at 400–410 nm and 750–950 nm correspond to the characteristic transitions of TCNQ˙⁻, while the bands at 370 nm and 600–700 nm can be attributed to the transitions of the TCNQ˙⁻ dimer.¹⁴ Furthermore, the broad absorption band at 1120 nm toward the NIR region suggests the formation of a charge-transfer (CT) state between P4VC_xP⁺ and TCNQ˙⁻. These results indicated that TCNQ˙⁻ coexisted in three distinct states in the pre-annealed film: monomeric, dimeric, and CT complexes. Annealing led to an overall decrease in absorption, with the absorption band at 680 nm derived from the TCNQ dimers remaining after 24 h. This behavior can be attributed to the dissociation of CT complexes and enhanced dimer formation, both of which resulted from changes in the higher-order structure caused by annealing. The overall decrease in the absorption can be explained by the end-on orientation of the π-conjugated systems. In addition, the transient increase followed by a decrease in absorption at 480 nm during annealing can be attributed to the partial oxidation of TCNQ˙⁻ and subsequent formation of the α,α-dicyano-p-toluoylcyanide anion (DCTC⁻) (Fig. S31).²⁵ The generation of DCTC⁻ was also confirmed by NMR. However, this unexpected oxidation did not affect the ion balance. Thus, we conclude that the π-conjugated anions also adopted an end-on conformation within the cylinders.

Fig. 5 UV-vis-NIR absorption spectra of (a) P4VC₁₀P⁺–Br⁻, (b) P4VC₁₀P⁺–In⁻, and (c) P4VC₁₀P⁺–TCNQ˙⁻ in the thin-film state. (d) The proposed schematic representation of P4VC_xP⁺ with π-conjugated anions.

Conclusions

P4VC_xP⁺–Br⁻ with linear alkyl chains (x = 10 or 14) were successfully synthesized. Subsequently, ion exchange between Br⁻ and In⁻ or TCNQ˙⁻ was performed to prepare P4VC_xP⁺–In⁻ and P4VC_xP⁺–TCNQ˙⁻, incorporating In⁻ and TCNQ˙⁻ as the counter anions, respectively. Humidity annealing was used to form higher-order structures of these ionic polymers. GI-SAXS analysis revealed microphase-separated structures, indicating that both the alkyl chain length of the cationic polymers and counter anion species influenced the resulting morphologies of the ionic polymers. For the P4VC_xP⁺–Br⁻ films, a hexagonal columnar structure was formed following humidity annealing for x = 10, whereas no significant structural change was observed for x = 14. Notably, P4VC_xP⁺–In⁻ and P4VC_xP⁺–TCNQ˙⁻, which contained In⁻ and TCNQ˙⁻ as counter anions, respectively, exhibited well-aligned hexagonal structures parallel to the glass substrate following humidity annealing. Furthermore, the π-conjugated anions adopted the end-on conformation within the cylinders. These results suggest that In⁻ and TCNQ˙⁻, which have larger in volumes than Br⁻ and exhibit π–π interactions, affect the higher-order structure of P4VC_xP⁺. Therefore, the microphase-separated morphology of ionic polymers can be effectively tuned by simply varying the alkyl chain length and the counter anion. Although still speculative, the detailed mechanism by which π-conjugated anions facilitate the rapid transformation from a random aggregate to a hexagonal columnar structure during humidity annealing requires elucidation in future studies.

Author contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: synthetic procedures, optical properties, thermal properties, XRD patterns, DFT calculations. See DOI: https://doi.org/10.1039/d5py00879d.

Data for this article, including synthetic procedures, analytical data, optical properties, and theoretical calculations, are available at https://doi.org/10.1039/x0xx00000x.

Acknowledgements

This work was supported by JSPS KAKENHI Grant Numbers JP21K14606, JP24K01292, JP24K21669, and JP24K08523, and JSPS Fellows Grant Numbers JP21J22405 and JP22KJ0332. The computation was performed using the Research Center for Computational Science, Okazaki, Japan (Project: 24-IMS-C093 and 25-IMS-C096). We thank Dr Hiromi Sekiguchi (ThermoFisher Scientific) for STEM measurements, and Prf. Seigo Kawaguchi and Dr Moriya Kikuchi (Yamagata University) for GPC measurements. The NMR analysis was performed using a JEOL JNM-EC500, JEOL Ltd introduced by the subsidy program for development of advanced research infrastructure. This work was partly supported by Program for Forming Japan's Peak Research Universities (J-PEAKS).

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